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Delivering on ultra-low power targets

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article image Clifford Shi, STMicroelectronics' Production & Application Manager for the MCU (microcontroller) product group in Australia and New Zealand.

WITH growing focus on smart home automation and other aspects of the "Internet of Things", how has one microcontroller innovator evolved to enable these capabilities?

In April 2014, STMicroelectronics announced the latest ultra-low-power (ULP) STM32L0 microcontrollers which offered engineers a small footprint on the board, while offering integrated features like embedded EEPROM, full RAM data retention, a low-power time counter, and fast wakeup time of 3.5 microseconds.

We talked to Clifford Shi, STMicroelectronics' Production & Application Manager for the MCU (microcontroller) product group in Australia and New Zealand, about the technology behind the company's latest ULP 32-bit MCUs.

Focus on IoT

Wearables, connected automotives, smart home, sensor hubs and interactive interfaces: these are all areas of major growth that rely on ever smaller devices that have the ability to connect to the Internet.

Besides the challenges of packing so much intelligence into a small space, electronic engineers must also figure out a way to provide these capabilities while using as little power as possible. Portable standalone devices have no access to mains power, and compact devices have limited room for batteries.

Shi says this is where STMicroelectronics comes in. Its ULP 32-bit MCUs allow product designers to deliver extended battery life in slim and lightweight devices

Process evolved

According to Shi, much of what is possible with STMicroelectronics' latest line of ULP 32-bit MCUs has to do with the 110nm ULP process that the company uses to fabricate the MCUs.

Standard 90nm node processes suffer from higher static current leakage, which increases exponentially with higher temperatures. The 110nm tune process, on the other hand, offers lower current static mode, while controlling leakage as temperature goes up. This allows the MCUs to remain operational even at higher temperatures (up to 125 degrees C).

The 110nm ULP process also allows for true embedded EEPROM, and the silicon does not cost more than the same product manufactured via the 90nm process.

Industrial capabilities

The STM32L0 MCUs aren't just for consumer devices. As indicated by the high operating temperature rating, they also have potential applications in industrial applications. According to Shi, built into the MCUs are features that would be highly applicable to industrial areas.

"The series features up to 64KB of Flash memory, up to 8KB of SRAM, and 2KB of embedded true EEPROM," Shi said.

"STM32L0 devices with integrated USB FS 2.0 interface support battery charger detection and link power management. Crystal-less operation of the USB is enabled via a built-in 48MHz Oscillator. All the lines offer built-in hardware engine encryption (AES) as an option."

Features of the MCUs include ADC, DAC, two ultra-low-power comparators, three 16-bit timers, one low-power timer (LPTIM), and one basic timer, one RTC, two watchdogs and one SysTick which can be used as time base.

"The STM32L0 series also embed standard and advanced communication interfaces: up to two I2Cs, two SPIs, two I2S, three USARTs and a crystal-less USB," Shi said. "Those features make STM32L0 series suited for many low power industrial applications."

Holistic approach to power sipping

The STM32L0 series has outstanding several low power modes including 139uA/MHz @ 32MHz, 87uA/MHz using an optimized low power mode, and 400nA in STOP mode with full RAM retention and a 3.5us fast wake-up time.

"The STM32L0 series also has the world’s lowest Analogue-to-Digital Converter (ADC) power consumption, Shi points out. "The 12-bit on-chip ADC draws only 48uA when operating at a 100ksamples/s."

"Those Ultra-Low-Power features make STM32L0 series particularly suited for devices that makes use of energy harvesting."

According to Shi, the MCUs' embedded peripherals like ULP ADCs and timers meant STMicroelectronics had to pay special attention to their design, because of their intrinsic high power consumption, or because they are always powere dup.

" The STM32L0 series embeds a 12-bit / 1.14MSps ADC. This very fast and accurate Analogue-to-Digital Converter can jeopardize the battery life time if left powered-up continuously," Shi said.

"As the ADC consumption is roughly proportional to the acquisition frequency, from low power consumption standpoint, the application can choose between two solutions, either performing the ADC acquisition at low speed to limit maximum current or doing it at maximum speed to switch in ultra-low power mode quickly."

The 16-bit Low Power Timer (LPTIM), on the other hand, is constantly running, but delivers value by providing power-saving capabilities.

"Thanks to its diversity of clock sources, the LPTIM is able to keep running whatever the selected power mode," Shi explained.

"Given its capability to run even with no external clock source, the LPTIM can be used as 'Pulse Counter' which can be useful in many applications. Also the LPTIM capability to wake up the system from low power modes, makes it suitable to realize 'Timeout Functions' with extremely low power consumption."

How to attain ultra low power consumption goals

The STM32L0 MCUs are easy to work with, thanks to the design tools supplied by STMicroelectronics, including a Power Consumption Calculator Wizard which helps designers reach ultra-low power consumption targets.

According to Shi, the power consumption of a microcontroller largely depends on two controllable factors: voltage and frequency.

"In the STM32L0 devices, an internal low drop regulator supplies most of the logic circuitry with a fixed voltage: this guarantees that consumption is kept minimal whatever the supply voltage, along the lifetime of portable battery-supplied products, down to 1.65V," Shi said.

"If we consider the clock sources, several cascaded clock prescalers, gating techniques and peripheral-by-peripheral clock management allow only the necessary logic gates to be activated, and at the adequate frequency. These are now design practices commonly used for reducing the consumption in RUN mode."

But the challenge isn't to focus on the RUN mode. To attain true ULP capabilities, the MCU needs to spend minimum time and energy in RUN mode, and find and enter the adequate low power mode.

To help with this challenge, STMicroelectronics enhanced the STM32L0 system with low power capabilities.

The system has seven low power modes optimized against current leakage (Sleep, Low Power Run, Low Power Sleep, Stop with RTC, Stop, Standby with RTC, Standby), and a set of low power peripherals tuned for low power (such as the calendar real-time clock, ADC, Low Power Timer, Low Power Comparators and glass LCD controller).

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